(1) Field of the Invention
The present invention relates to two-piece solid golf balls having a particularly improved initial velocity, comprising a solid core, which comprises polybutadiene, a polyfunctional unsaturated ester and an unsaturated carboxylic acid which is enclosed with a cover comprising a specifically limited metal salt of a copolymer of ethylene and unsaturated carboxylic acid.
(2) Description of the Prior Art
The carry of the golf ball is influenced by various factors, such as initial velocity when the ball is struck by a golf club, rotation frequency, driven angle, weight, radius, moment of inertia, shape of dimple and the like. When theoretically other conditions are constant, but the initial velocity of golf ball is higher, the carry thereof is longer. Therefore, it is desirable to increase the initial velocity of golf ball in order to improve its performance.
Solid golf balls were first proposed by J. S. Bartsch, and there have hitherto been proposed a large number of solid golf balls. However, these solid golf balls are inferior to any conventional wound golf ball in carry and other properties, and therefore the development of solid golf balls free from such drawbacks has been demanded. For example, a unitary solid golf ball comprising a polyfunctional unsaturated ester and methacrylic acid is disclosed in Japanese Patent Laid Open Application No. 48,423/74 (corresponding to U.S. patent application Ser. No. 250,147 now U.S. Pat. No. 4,056,269), but the golf ball is still insufficient in initial velocity. Further, a two-piece solid golf ball comprising a solid core, which comprises a polyfunctional unsaturated ester, enclosed with a cover of a metal salt of an ethylene/unsaturated carboxylic acid copolymer, is disclosed in Japanese Patent Application Publication No. 9,257/72, but the golf ball is low in initial velocity and poor in durability and further is too soft to give a good click and feel to a player, that is, the golf ball is not satisfactory for use in tournaments and competitions. It is necessary that golf balls for tournaments be well balanced in performance, such as carry, durability and click and feel, which cannot be expressed quantitatively. However, both of the conventional unitary and two-piece solid golf balls are still insufficient in some of the above described performance factors.